Several in vitro systems were explored for their applicability to the testing of (suspected) neurotoxic substances, such as Erythrosin B (FD and C Red 3) (EB) an artificial halogenated fluorescein derivative. The dye can be used to trace solubilization and partial purification of rat brain cortex ATPase. It also inhibits ATPase and calcium transport activity of rabbit muscle sarcoplasmic reticulum. EB inhibits axonal outgrowth from chick dorsal root explants by photo-oxidizing NGF. General in vitro toxic effects are also seen. Storage and release of catecholamines from adrenal medullary cells are affected by a variety of heavy metals, partially through interference with calcium-specific mechanisms involved in release of the neurotransmitter. The calcium-promoted fusion of isolated chromaffin granules, and its inhibition by various heavy metals, is being studied as a model process for exocytotic release of catecholamines in vivo. The kinetics of calcium-promoted aggregation and fusion of the granules have been studied using fluorescence energy transfer techniques.